A plasma processing apparatus for performing a plasma processing on a wafer serving as a substrate includes a processing chamber for accommodating the wafer therein, a mounting table for mounting thereon the wafer in the processing chamber, a shower head for supplying a processing gas into the processing chamber, which is provided to face the mounting table. In the plasma processing apparatus, high frequency power sources for plasma generation and for plasma attraction are connected to the mounting table.
More specifically, the high frequency voltage for plasma generation is applied to the mounting table in the processing chamber, by which a processing gas supplied into the processing chamber is excited to generate a plasma (i.e., positive ions and/or electrons). The generated positive ions and electrons are attracted to the wafer mounted on the mounting table by the high frequency voltage (referred to as a ‘bias voltage’ hereinafter) which is applied thereto. The wafer is physically etched by the positive ions attracted thereto.
In recent years, by achieving an increased electron density within the processing chamber to thereby increase the number of electrons arriving at the wafer, there has been made close investigations on the possibility for aggressively employing electrons to perform processes on the wafer, e.g., reforming process of a photoresist film on the wafer or alleviating process of shading effect.
There is known a plasma processing apparatus in which a direct voltage is applied to a shower head, in order to increase an electron density in a processing chamber (see, e.g., Japanese Application Publication No. 2006-270019). In this plasma processing apparatus, a direct current (DC) power source is connected to a disk-like upper electrode which is included as a component of the shower head and exposed in the processing chamber.
Herein, if a negative DC voltage is applied to the upper electrode, only positive ions in the plasma will be attracted to the upper electrode. Since a potential is a constant in the DC voltage unlike as in the high frequency voltage, the positive ions are continuously attracted to the upper electrode to thereby collide therewith. Secondary electrons are emitted from atoms in the upper electrode by the positive ions colliding therewith, thereby increasing the electron density in the processing chamber.
Further, as the mounting table is applied with a bias voltage, a sheath is generated above the wafer, which is a region of high electron density. The positive ions are accelerated toward the wafer by the sheath, thereby physically etching it.
However, since the sheath repels electrons, it becomes difficult for the secondary electrons emitted from the upper electrode to arrive at the wafer. In other words, if the mounting table is applied with the bias voltage in order to perform an etching process onto the wafer, the electrons cannot be used to carry out a process on the wafer.